Solid-state lighting arrays are used for a number of lighting applications. For example, solid-state lighting panels including arrays of solid-state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid-state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers.
Solid-state lighting arrays are used for a number of lighting applications. For example, solid-state lighting panels including arrays of solid-state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. Solid-state lighting devices are also used in lighting fixtures, such as incandescent bulb replacement applications, task lighting, recessed light fixtures and the like. For example, Cree, Inc. produces a variety of recessed downlights, such as the LR-6 and CR-6, which use LEDs for illumination. Solid-state lighting panels are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices, and for larger displays, such as LCD television displays.
A solid-state light emitting device may include, for example, a packaged light emitting device including one or more LEDs. Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid-state light emitting device. Typically, a solid-state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
Some attempts at providing solid-state lighting sources have involved driving an LED or string or group of LEDs using a rectified alternating current (ac) waveform. However, because the LEDs require a minimum forward voltage to turn on, the LEDs may turn on for only a part of the rectified ac waveform, which may result in visible flickering, may undesirably lower the power factor of the system, and/or may increase resistive loss in the system. Examples of techniques for driving LEDs with a rectified ac waveform are described in U.S. Patent Application Publication No. 2010/0308738 and in copending U.S. patent application Ser. No. 12/777,842, the latter of which is commonly assigned to the assignee of the present application.
Other attempts at providing ac-driven solid-state lighting sources have involved placing LEDs in an anti-parallel configuration, so that half of the LEDs are driven on each half-cycle of an ac waveform. However, this approach requires twice as many LEDs to produce the same luminous flux as using a rectified ac signal.